U.S. patent number 6,797,217 [Application Number 10/242,160] was granted by the patent office on 2004-09-28 for methods for making encapsulated stent-grafts.
This patent grant is currently assigned to Bard Peripheral Vascular, Inc.. Invention is credited to Christopher E. Banas, Tarun J. Edwin, Brendan J. McCrea.
United States Patent |
6,797,217 |
McCrea , et al. |
September 28, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Methods for making encapsulated stent-grafts
Abstract
Shape memory alloy and elastically self-expanding endoluminal
stents which are at least partially encapsulated in a substantially
monolithic expanded polytetrafluorethylene ("ePTFE") covering. An
endoluminal stent, which has a reduced diametric dimension for
endoluminal delivery and a larger in vivo final diametric diameter
is encapsulated in an ePTFE covering which circumferentially covers
both the luminal and abluminal walls along at least a portion of
the longitudinal extent of the endoluminal stent. The endoluminal
stent is fabricated from a shape memory alloy which exhibits either
shape memory or pseudoelastic properties or from an elastic
material having an inherent spring tension.
Inventors: |
McCrea; Brendan J. (Ballwin,
MO), Edwin; Tarun J. (Chandler, AZ), Banas; Christopher
E. (San Antonio, TX) |
Assignee: |
Bard Peripheral Vascular, Inc.
(Tempe, AZ)
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Family
ID: |
46255796 |
Appl.
No.: |
10/242,160 |
Filed: |
September 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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833797 |
Apr 9, 1997 |
6451047 |
|
|
|
508033 |
Jul 27, 1995 |
5749880 |
|
|
|
401871 |
Mar 10, 1995 |
6124523 |
|
|
|
795871 |
Feb 5, 1997 |
6039755 |
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Current U.S.
Class: |
264/229; 264/230;
264/249; 264/266; 264/273; 264/294; 623/901 |
Current CPC
Class: |
A61F
2/06 (20130101); A61F 2/07 (20130101); A61L
27/507 (20130101); A61L 31/048 (20130101); A61L
31/10 (20130101); B29C 55/005 (20130101); A61L
31/048 (20130101); C08L 27/18 (20130101); A61L
31/10 (20130101); C08L 27/18 (20130101); A61F
2/82 (20130101); A61F 2002/072 (20130101); A61L
2400/16 (20130101); B29C 55/22 (20130101); B29K
2027/18 (20130101); Y10S 623/901 (20130101); A61F
2/90 (20130101) |
Current International
Class: |
A61F
2/06 (20060101); A61L 27/00 (20060101); A61L
27/50 (20060101); A61L 31/04 (20060101); B29C
061/02 (); B29C 061/04 (); B29C 065/02 () |
Field of
Search: |
;264/229,273,274,275,259,254,250,271.1,319,320,230,294
;623/1.13,909,1.11,1.12,1.27,1.44,11.11,901 ;606/191,194,195,198
;156/196,293 |
References Cited
[Referenced By]
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WO |
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Acta 162:189-193..
|
Primary Examiner: Ortiz; Angela
Attorney, Agent or Firm: Morrison & Foerster, LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a division of application Ser. No. 08/833,797,
filed Apr. 9, 1997, now U.S. Pat. No. 6,451,047, which is a
continuation-in-part of two applications: 1) Ser. No. 08/508,033,
filed Jul. 27, 1995, now U.S. Pat. No. 5,749,880, which is a
continuation-in-part of application Ser. No. 08/401,871, filed Mar.
10, 1995, now U.S. Pat. No. 6,124,523; and 2) Ser. No. 08/794,871,
filed Feb. 5, 1997, now U.S. Pat. No. 6,039,755. This application
expressly incorporates by reference the entirety of each of the
above-mentioned applications as if fully set forth herein.
Claims
What is claimed is:
1. A method for making an encapsulated stent-graft, the stent-graft
comprising a self-expanding stent having an essentially tubular
configuration with a central longitudinal lumen and having a first
diameter and a second diameter, wherein the first diameter is
larger than the second diameter, comprising the steps of: placing a
first tube of biocompatible material over a mandrel; manipulating
said stent from said first diameter to said second diameter;
concentrically engaging said stent about first tube at said second
diameter; concentrically engaging a second tube of biocompatible
material about said stent and said first tube, forming a
stent-graft assembly; applying pressure to said assembly by winding
a layer of tape over said second tube to compress said assembly
against said mandrel; and heating said assembly, wherein said first
tube is joined to said second tube through openings in the wall of
said stent, forming a monolithic layer of biocompatible material
that applies a constraining force on said stent, preventing said
stent from expanding to said first diameter, the monolithic layer
of biocompatible material being radially deformable to release said
constraining force on said stent.
2. The method according to claim 1, wherein the biocompatible
material in said first and second tubes comprises seamless expanded
polytetrafluoroethylene having a node-fibril microstructure.
3. The method according to claim 2, wherein at least one of said
first and second tubes is unsintered.
4. The method according to claim 2, wherein said node-fibril
microstructure of said first and second tubes contains fibrils
having a parallel orientation with respect to the central
longitudinal lumen of the stent.
5. The method according to claim 2, wherein said node-fibril
microstructure of said first and second tubes contains fibrils
having a perpendicular orientation with respect to the central
longitudinal lumen of the stent.
6. The method according to claim 1, wherein said stent is comprised
of shape memory alloy having an Austenite phase and a Martensite
phase, the manipulating step further comprising cooling said stent
to a temperature below the martensitic transformation temperature
thereof.
7. The method according to claim 1, wherein said stent is comprised
of shape memory alloy having an Austenite phase and a Martensite
phase, and wherein the manipulating step is performed at a
temperature above the martensitic transformation temperature of the
stent.
8. A method for making an encapsulated stent-graft, the stent-graft
comprising a self-expanding stent having an essentially tubular
configuration with a central longitudinal lumen and having a first
diameter and a second diameter, wherein the first diameter is
larger than the second diameter, comprising the steps of: placing a
first tube of biocompatible material over a mandrel; concentrically
engaging said stent about first tube at said first diameter;
concentrically engaging a second tube of biocompatible material
about said stent and said first tube, forming a stent-graft
assembly; applying pressure to said assembly by winding a layer of
tape over said second tube to compress said assembly against said
mandrel; heating said assembly, wherein said first tube is joined
to said second tube through openings in the wall of said stent,
forming a monolithic layer of biocompatible material; and
manipulating said assembly to said second diameter, wherein a
constraining force in the form of a delivery sheath is applied to
said assembly, preventing said assembly from expanding to said
first diameter.
9. The method according to claim 8, wherein the biocompatible
material in said first and second tubes comprises seamless expanded
polytetrafluoroethylene having a node-fibril microstructure.
10. The method according to claim 9, wherein at least one of said
first and second tubes is unsintered.
11. The method according to claim 9, wherein said node-fibril
microstructure of said first and second tubes contains fibrils
having a parallel orientation with respect to the central
longitudinal lumen of the stent.
12. The method according to claim 9, wherein said node-fibril
microstructure of said first and second tubes contains fibrils
having a perpendicular orientation with respect to the central
longitudinal lumen of the stent.
13. The method according to claim 8, wherein said stent is
comprised of shape memory alloy having an Austenite phase and a
Martensite phase, the manipulating step further comprising cooling
said assembly to a temperature below the martensitic transformation
temperature of the stent.
14. The method according to claim 8, wherein said stent is
comprised of shape memory alloy having an Austenite phase and a
Martensite phase, and wherein the manipulating step is performed at
a temperature above the martensitic transformation temperature of
the stent.
15. A method for making an encapsulated stent-graph, the
stent-graph comprising a self-expanding stent comprised of shape
memory alloy having an Austenite phase and a Martensite phase, and
having an essentially tubular configuration with a central
longitudinal lumen and a first diameter and a second diameter,
wherein the first diameter is larger than the second diameter,
comprising the steps of: manipulating said stent from said first
diameter to said second diameter, wherein said manipulating
comprises cooling said stent to a temperature below the martensitic
transformation temperature thereof; concentrically engaging said
stent about a first tube of biocompatible material at said second
diameter; concentrically engaging a second tube of biocompatible
material about said stent and said first tube, forming a
stent-graft assembly; applying pressure to at least one of said
first and second tubes; and heating said assembly, wherein said
first tube is joined to said second tube through openings in the
wall of said stent, forming a monolithic layer of biocompatible
material that applies a constraining force on said stent,
preventing said stent from expanding to said first diameter, the
monolithic layer of biocompatible material being radially
deformable to release said constraining force on said stent.
16. A method for making an encapsulated stent-graph, the
stent-graph comprising a self-expanding stent comprised of shape
memory alloy having an Austenite phase and a Martensite phase, and
having an essentially tubular configuration with a central
longitudinal lumen and a first diameter and a second diameter,
wherein the first diameter is larger than the second diameter,
comprising the steps of: manipulating said stent from said first
diameter to said second diameter at a temperature above the
martensitic transformation temperature of said stent;
concentrically engaging said stent about a first tube of
biocompatible material at said second diameter; concentrically
engaging a second tube of biocompatible material about said stent
and said first tube, forming a stent-graft assembly; applying
pressure to at least one of said first and second tubes; and
heating said assembly, wherein said first tube is joined to said
second tube through openings in the wall of said stent, forming a
monolithic layer of biocompatible material that applies a
constraining force on said stent, preventing said stent from
expanding to said first diameter, the monolithic layer of
biocompatible material being radially deformable to release said
constraining force on said stent.
17. A method for making an encapsulated stent-graph, the
stent-graph comprising a self-expanding stent comprised of shape
memory alloy having an Austenite phase and a Martensite phase, and
having an essentially tubular configuration with a central
longitudinal lumen and a first diameter and a second diameter,
wherein the first diameter is larger than the second diameter,
comprising the steps of: concentrically engaging said stent about a
first tube of biocompatible material at said first diameter;
concentrically engaging a second tube of biocompatible material
about said stent and said first tube, forming a stent-graft
assembly; applying pressure to at least one of said first and
second tubes; heating said assembly, wherein said first tube is
joined to said second tube through openings in the wall of said
stent, forming a monolithic layer of biocompatible material; and
manipulating said assembly to said second diameter, wherein said
manipulating comprises cooling said stent to a temperature below
the martensitic transformation temperature thereof, wherein a
constraining force in the form of a delivery sheath is applied to
said assembly, preventing said assembly from expanding to said
first diameter.
18. A method for making an encapsulated stent-graph, the
stent-graph comprising a self-expanding stent comprised of shape
memory alloy having an Austenite phase and a Martensite phase, and
having an essentially tubular configuration with a central
longitudinal lumen and a first diameter and a second diameter,
wherein the first diameter is larger than the second diameter,
comprising the steps of: concentrically engaging said stent about a
first tube of biocompatible material at said first diameter;
concentrically engaging a second tube of biocompatible material
about said stent and said first tube, forming a stent-graft
assembly; applying pressure to at least one of said first and
second tubes; heating said assembly, wherein said first tube is
joined to said second tube through openings in the wall of said
stent, forming a monolithic layer of biocompatible material; and
manipulating said assembly to said second diameter at a temperature
above the martensitic transformation temperature of said stent,
wherein a constraining force in the form of a delivery sheath is
applied to said assembly, preventing said assembly from expanding
to said first diameter.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to implantable intraluminal
devices, particularly intraluminal stents. Because of the open
lattice found in most intraluminal stents, a primary problem with
these types of devices is occlusion of the vessel occurring after
stent placement. Tissue ingrowth and neointimal hyperplasia
significantly reduces the open diameter of the treated lumen over
time, requiring additional therapies. The present invention
incorporates the use of a biocompatible barrier material that
prevents or delays the tissue ingrowth and neointimal hyperplasia,
thus maintaining luminal patency for longer periods after initial
treatment. The use of expanded polytetrafluoroethylene (ePTFE) as a
bio-inert barrier material is well documented. In accordance with
certain of its preferred embodiments, the present invention
utilizes a radially expandable ePTFE material, such as that
described in U.S. Pat. No. 6,039,755, to partially or fully embed
the stent lattice, thereby providing a suitable barrier which
improves stent patency.
The inventive intraluminal stent-graft device may be implanted
either by percutaneous delivery using an appropriate delivery
system, a cut-down procedure in which a surgical incision is made
and the intraluminal device implanted through the surgical
incision, or by laparoscopic or endoscopic or endoscopic delivery.
More particularly the present invention relates to shape memory
alloy and self-expanding endoluminal stents which are at least
partially encapsulated in a substantially monolithic expanded
polytetrafluoroethylene ("ePTFE") covering. In accordance with the
present invention, an endoluminal stent, which has a reduced
diametric dimension for endoluminal delivery and a larger in vivo
final diametric diameter, is encapsulated in an ePTFE covering
which circumferentially covers both the luminal and abluminal walls
along at least a portion of the longitudinal extent of the
endoluminal stent. The endoluminal stent is preferably fabricated
from a shape memory alloy which exhibits either shape memory or
pseudoelastic properties or from an elastic material having an
inherent spring tension.
In a first embodiment of the invention, the endoluminal stent is
encapsulated in the ePTFE covering in the stent's reduced diametric
dimension and is balloon expanded in vivo to radially deform the
ePTFE covering. The endoluminal stent may be either one which
exhibits thermal strain recovery, pseudoelastic stress-strain
behavior or elastic behavior at mammalian body temperature. While
in its reduced diametric dimension the ePTFE encapsulating covering
integrally constrains the endoluminal stent from exhibiting either
thermal strain recovery, pseudoelastic stress-strain behavior or
elastic behavior at mammalian body temperature. Radial deformation
of the ePTFE covering releases constraining forces acting on the
endoluminal stent by the undeformed ePTFE covering and permits the
stent to radially expand.
In a second embodiment of the invention, an endoluminal stent
fabricated of a shape memory alloy is encapsulated in its final
diametric dimension and the encapsulated intraluminal stent-graft
is manipulated into its reduced diametric dimension and radially
expanded in vivo under the influence of a martensite to austenite
transformation.
In a third embodiment of the present invention, a self-expanding
intraluminal stent, fabricated of a material having an inherent
spring tension, is encapsulated in its final diametric dimension
and manipulated to a reduced diametric dimension and externally
constrained for intraluminal delivery. Upon release of the external
constraint in vivo the spring tension exerted by the self-expanding
stent radially expands both the stent and the ePTFE encapsulating
covering to a radially enlarged diameter.
In a fourth embodiment of the invention, the endoluminal stent is
fabricated from a material having an inherent elastic spring
tension and is encapsulated at a reduced dimension suitable for
endoluminal delivery and balloon expanded in vivo to radially
deform the ePTFE covering.
Shape memory alloys are a group of metallic materials that
demonstrate the ability to return to a defined shape or size when
subjected to certain thermal or stress conditions. Shape memory
alloys are generally capable of being plastically deformed at a
relatively low temperature and, upon exposure to a relatively
higher temperature, return to the defined shape or size prior to
the deformation. Shape memory alloys may be further defined as one
that yields a thermoplastic martensite. A shape memory alloy which
yields a thermoplastic martensite undergoes a martensitic
transformation of a type that permits the alloy to be deformed by a
twinning mechanism below the martensitic transformation
temperature. The deformation is then reversed when the twinned
structure reverts upon heating to the parent austenite phase. The
austenite phase occurs when the material is at a low strain state
and occurs at a given temperature. The martensite phase may be
either temperature-induced martensite (TIM) or stress-induced
martensite (SIM).
When a shape memory material is stressed at a temperature above the
start of martensite formation, denoted M.sub.s, where the
austenitic state is initially stable, but below the maximum
temperature at which martensite formation can occur, denoted
M.sub.d, the material first deforms elastically and when a critical
stress is reached, it begins to transform by the formation of
stress-induced martensite. Depending upon whether the temperature
is above or below the start of austenite formation, denoted
A.sub.s, the behavior when the deforming stress is released
differs. If the temperature is below A.sub.s, the stress-induced
martensite is stable; however, if the temperature is above A.sub.s,
the martensite is unstable and transforms back to austenite, with
the sample returning to its original shape. U.S. Pat. Nos.
5,597,378, 5,067,957 and 4,665,906 disclose devices, including
endoluminal stents, which are delivered in the stress-induced
martensite phase of shape memory alloy and return to their
pre-programmed shape by removal of the stress and transformation
from stress-induced martensite to austenite.
Shape memory characteristics may be imparted to a shape memory
alloy by heating the metal at a temperature above which the
transformation from the martensite phase to the austenite phase is
complete, i.e., a temperature above which the austenite phase is
stable. The shape of the metal during this heat treatment is the
shape "remembered." The heat treated metal is cooled to a
temperature at which the martensite phase is stable, causing the
austenite phase to transform to the martensite phase. The metal in
the martensite phase is then plastically deformed, e.g., to
facilitate its delivery into a patient's body. Subsequent heating
of the deformed martensite phase to a temperature above the
martensite to austenite transformation temperature, e.g., body
temperature, causes the deformed martensite phase to transform to
the austenite phase and during this phase transformation, the metal
reverts back to its original shape.
The term "shape memory" is used in the art to describe the property
of a material to recover a pre-programmed shape after deformation
of a shape memory alloy in its martensitic phase and exposing the
alloy to a temperature excursion through its austenite
transformation temperature, at which temperature the alloy begins
to revert to the austenite phase and recover its preprogrammed
shape. The term "pseudoelasticity" is used to describe a property
of shape memory alloys where the alloy is stressed at a temperature
above the transformation temperature of the alloy and
stress-induced martensite is formed above the normal martensite
formation temperature. Because it has been formed above its normal
temperature, stress-induced martensite reverts immediately to
undeformed austenite as soon as the stress is removed, provided the
temperature remains above the transformation temperature.
The martensitic transformation that occurs in the shape memory
alloys yields a thermoplastic martensite and develops from a
high-temperature austenite phase with long-range order. The
martensite typically occurs as alternately sheared platelets, which
are seen as a herringbone structure when viewed metallographically.
The transformation, although a first-order phase change, does not
occur at a single temperature but over a range of temperatures that
varies with each alloy system. Most of the transformation occurs
over a relatively narrow temperature range, although the beginning
and end of the transformation during heating or cooling actually
extends over a much larger temperature range. The transformation
also exhibits hysteresis in that the transformations on heating and
on cooling do not overlap. This transformation hysteresis varies
with the alloy system.
A thermoplastic martensite phase is characterized by having a low
energy state and glissile interfaces, which can be driven by small
temperature or stress changes. As a consequence of this, and of the
constraint due to the loss of symmetry during transformation, a
thermoplastic martensite phase is crystallographically reversible.
The herringbone structure of athermal martensite essentially
consists of twin-related, self-accommodating variants. The shape
change among the variants tends to cause them to eliminate each
other. As a result, little macroscopic strain is generated. In the
case of stress-induced martensite, or when stressing a
self-accommodating structure, the variant that can transform and
yield the greatest shape change in the direction of the applied
stress is stabilized and becomes dominant in the configuration.
This process creates a macroscopic strain, which is recoverable as
the crystal structure reverts to austenite during reverse
transformation.
The mechanical properties of shape memory alloys vary greatly over
the transformation temperature range. Martensite phase alloys may
be deformed to several percent strain at quite a low stress,
whereas the austenite phase alloy has much higher yield and flow
stresses. Upon heating after removing the stress, the martensite
phase shape memory alloy will remember its unstrained shape and
revert to its austenite phase.
Where a shape memory alloy is exposed to temperature above its
transformation temperature, the martensite phase can be
stress-induced. Once stress-induced martensite occurs, the alloy
immediately strains and exhibits the increasing strain at constant
stress behavior. Upon unloading of the strain however, the shape
memory alloy reverts to austenite at a lower stress and shape
recovery occurs, not upon the application of heat but upon a
reduction of stress. This effect, which causes the material to be
extremely elastic, is known as pseudoelasticity and the effect is
nonlinear.
The present invention preferably utilizes an binary, equiatomic
nickel-titanium alloy because of its biocompatibility and because
such an alloy exhibits a transformation temperature within the
range of physiologically-compatible temperatures. Nickel-titanium
alloys exhibit moderate solubility for excess nickel or titanium,
as well as most other metallic elements, and also exhibits a
ductility comparable to most ordinary alloys. This solubility
allows alloying with many of the elements to modify both the
mechanical properties and the transformation properties of the
system. Excess nickel, in amounts up to about 1%, is the most
common alloying addition. Excess nickel strongly depresses the
transformation temperature and increases the yield strength of the
austenite. Other frequently used elements are iron and chromium (to
lower the transformation temperature), and copper (to decrease the
hysteresis and lower the deformation stress of the martensite).
Because common contaminants such as oxygen and carbon can also
shift the transformation temperature and degrade the mechanical
properties, it is also desirable to minimize the amount of these
elements.
As used in this application, the following terms have the following
meanings:
A.sub.f Temperature: The temperature at which a shape memory alloy
finishes transforming to Austenite upon heating.
A.sub.s Temperature: The temperature at which a shape memory alloy
starts transforming to Austenite upon heating.
Austenite: The stronger, higher temperature phase present in
NiTi.
Hysteresis: The temperature difference between a phase
transformation upon heating and cooling. In NiTi alloys, it is
generally measured as the difference between A.sub.p and
M.sub.p.
M.sub.f Temperature: The temperature at which a shape memory alloy
finishes transforming to Martensite upon cooling.
M.sub.s Temperature: The temperature at which a shape memory alloy
starts transforming to Martensite upon cooling.
Martensite: The more deformable, lower temperature phase present in
NiTi.
Phase Transformation: The change from one alloy phase to another
with a chsnge in temperature, pressure, stress, chemistry, and/or
time.
Shape Memory: The ability of certain alloys to return to a
predetermined shape upon heating via a phase transformation.
Pseudoelasticity: The reversible non-linear elastic deformation
obtained when austenitic shape memory alloys are strained at a
temperature above A.sub.s, but below M.sub.d, the maximum
temperature at which pseudoelasticity is obtained.
Thermoplastic Martensitic Transformation: A diffusionless,
thermally reversible phase transformation characterized by a
crystal lattice distortion.
BRIEF SUMMARY OF THE INVENTION
It is a principal objective of the present invention to encapsulate
an intraluminal structural support with a substantially monolithic
covering of ePTFE.
It is a further objective of the present invention to encapsulate a
shape memory alloy intraluminal stent with a substantially
monolithic covering of ePTFE.
It is another object of the present invention to provide a unique
library of endoprostheses consisting generally of intraluminal
structural supports made of shape memory alloys, which are at least
partially encapsulated in a substantially monolithic expanded
polytetrafluoroethylene covering, and which exhibit either thermal
strain recovery, pseudoelastic stress-strain behavior or elastic
behavior at mammalian body temperature.
It is a further objective of the present invention to encapsulate a
shape memory alloy intraluminal stent at a reduced delivery
diametric dimension and balloon expand the ePTFE encapsulated
stent-graft to radially deform the ePTFE covering and release the
radial constraint exerted by the ePTFE encapsulation on the shape
memory stent thereby permitting the shape memory alloy stent to
undergo transformation from its radially constrained dimension to
an enlarged deployed dimension.
It is another objective of the present invention to encapsulate a
shape memory alloy intraluminal stent at its enlarged diametric
dimension, either with an at least partially unsintered tubular
ePTFE extrudate having a diametric dimension comparable to the
enlarged diametric dimension of the shape memory alloy intraluminal
stent, or with a fully sintered ePTFE tubular member which has been
radially expanded to a diametric dimension comparable to the
enlarged diametric dimension of the shape memory alloy intraluminal
stent, where the encapsulated intraluminal stent is then reduced in
its diametric dimension for endoluminal delivery.
It is yet a further objective of the present invention to
encapsulate a self-expanding intraluminal stent, such as a
Gianturco stent or a pseudoelastic shape memory stent, at a reduced
delivery diametric dimension and balloon expand the ePTFE
encapsulated stent-graft to radially deform the ePTFE covering and
release the radial constraint exerted by the ePTFE encapsulation on
the self-expanding stent thereby permitting the self-expanding
stent to elastically radially expand to its in vivo diameter.
It is another objective of the present invention to encapsulate a
self-expanding intraluminal stent at its enlarged diametric
dimension, either with an at least partially unsintered tubular
ePTFE extrudate having a diametric dimension comparable to the
enlarged diametric dimension of the shape memory alloy intraluminal
stent, or with a fully sintered ePTFE tubular member which has been
radially expanded to a diametric dimension comparable to the
enlarged diametric dimension of the self-expanding intraluminal
stent, and reducing the diametric dimension of the encapsulated
stent for endoluminal delivery.
It is a still further objective of the present invention to
encapsulate at a reduced delivery diametric dimension and balloon
expand the ePTFE encapsulated stent-graft to radially deform the
ePTFE covering and release the radial constraint exerted by the
ePTFE encapsulation on the shape memory stent thereby permitting
the stent to radially expand to a larger in vivo diametric
dimension either by the shape memory property of the stent material
or by elastic spring tension.
It is a further objective of the present invention to provide
methods of encapsulating shape memory alloy intraluminal stents and
self-expanding intraluminal stents, either at their reduced
diametric dimension or at their in vivo diametric dimension.
It is another objective of the present invention to provide an
ePTFE encapsulated intraluminal stent which is encapsulated between
luminal and abluminal ePTFE tubular members, where the ePTFE
tubular members may be applied to the intraluminal stent in their
unsintered, partially sintered or fully sintered state.
It is a further objective of the present invention to employ an
ePTFE interlayer positioned adjacent either the luminal or the
abluminal surface of the intraluminal stent as a bonding adjuvant
interlayer between the luminal and abluminal ePTFE tubular
members.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the ePTFE encapsulated intraluminal
stent in accordance with the present invention.
FIG. 2A is a perspective view of an ePTFE encapsulated intraluminal
stent encapsulated at its reduced diametric dimension for
intraluminal delivery and balloon assisted expansion in vivo.
FIG. 2B is a perspective view of the ePTFE encapsulated
intraluminal stent of FIG. 2A illustrating the ePTFE encapsulated
intraluminal stent after balloon assisted expansion.
FIG. 3 is a side elevational, longitudinal cross-sectional view of
the inventive ePTFE encapsulated intraluminal stent encapsulated at
its nominal in vivo dimension.
FIG. 4 is a side elevational, longitudinal cross-sectional view of
the inventive ePTFE encapsulated stent, illustrating the ePTFE
encapsulated intraluminal stent partially at a reduced intraluminal
delivery diameter deformed to a relatively reduced diameter
suitable for intraluminal delivery, mounted on a delivery catheter
having an axially moveable constraining sheath which constrains the
ePTFE encapsulated self-expanding intraluminal stent in its
relatively reduced diametric dimension.
FIG. 5 is a scanning electron micrograph, taken at 300.times.
magnification, of an outer surface of the radially expanded ePTFE
material used to encapsulate the balloon assisted radially
expandable encapsulated Nitinol stent of the present invention.
FIG. 6 is a scanning electron micrograph, taken at 300.times.
magnification, of an inner surface of the ePTFE material used to
encapsulate the balloon assisted radially expandable encapsulate
Nitinol stent of the present invention.
FIG. 7 is a scanning electron micrograph, taken at 300.times.
magnification, of an outer surface of the ePTFE material used to
encapsulate a self-expanding Nitinol or spring tension stent in
accordance with the present invention.
FIG. 8 is a scanning electron micrograph, taken at 300.times.
magnification, of an inner surface of the ePTFE material used to
encapsulate a self-expanding Nitinol or spring tension stent of the
present invention.
FIG. 9A is a flow diagram illustrating the inventive process steps
to thermomechanically deform a pre-programmed shape memory stent to
a reduced diametric dimension for encapsulation or endoluminal
delivery.
FIG. 9B is a flow diagram illustrating the inventive process steps
to encapsulate a shape memory alloy stent and a self-expanding
stent to make each preferred embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates the ePTFE encapsulated intraluminal stent 10 of
the present invention in a radially enlarged diametric dimension.
The inventive ePTFE encapsulated intraluminal stent 10 of the
present invention is best illustrated with reference to several
preferred embodiments thereof. The first preferred embodiment is
depicted in FIGS. 2A-2B and consists generally of an intraluminal
stent 12 made of a shape memory alloy which is at least partially
encapsulated in a substantially monolithic ePTFE covering 14 while
in a relatively smaller diametric dimension D.sub.1 and is radially
expandable in vivo under the influence of a radially outwardly
directed force which radially deforms the ePTFE covering 14 and
releases the stress exerted on the intraluminal stent 12 while at
body temperature to permit the intraluminal stent to undergo
deformation to a larger diametric dimension D.sub.2.
FIGS. 3-4 generically depict the second, third and fourth preferred
embodiments of the present invention an intraluminal stent 20 which
is at least partially encapsulated within a substantially
monolithic ePTFE covering 14 over at least an entire
circumferential portion of the luminal and abluminal surfaces of
the intraluminal stent 12. The second, third and fourth preferred
embodiments differ from one another based upon the type of
intraluminal stent 20 utilized and whether the encapsulated stent
device is intended to radially expand in vivo under the influence
of the shape memory behavior or elastic spring tension behavior of
the intraluminal stent 20 or whether in vivo delivery will be
balloon catheter assisted. Optionally, as discussed in the fifth
preferred embodiment, an interlayer member made of at least
partially unsintered ePTFE, such as ring-like member 23, may be
interdisposed between an inner and outer ePTFE layer adjacent the
intraluminal stent 20 to assist with the adhesion of the layers
and/or to serve as a barrier between a radiopaque marker (not
shown) and the intraluminal stent 20.
The second preferred embodiment of the present invention consists
generally of an intraluminal stent 20 made of a shape memory alloy,
which is at least partially encapsulated in a substantially
monolithic ePTFE covering 14 while in a relatively larger diametric
dimension D.sub.2 and in the austenite phase. The at least
partially encapsulated stent is then thermomechanically deformed at
a temperature induced martensite phase to a smaller diametric
dimension D.sub.1 and is constrained by a constraining sheath 22
for endoluminal delivery. Once at the delivery site, the external
constraint 22 is removed and the intraluminal stent 20 undergoes
martensitic transformation to the austenite state and
thermoelastically deforms 24 to its predetermined shape while
unfolding or decompressing, without plastically deforming, the
ePTFE covering 14, making contact with the luminal tissue (not
shown).
The third preferred embodiment of the present invention consists
generally of a self-expanding intraluminal stent 20 made from
either an elastic spring material or of a pseudoelastic shape
memory material, and is at least partially encapsulated in a
substantially monolithic ePTFE covering 14 while in a relatively
small diametric dimension D.sub.1 such that the ePTFE encapsulating
covering 14 acts to impart strain upon the intraluminal stent 20
and constrain the intraluminal stent from radial expansion to a
relatively larger diametric dimension D.sub.2 until intraluminally
delivered, wherein the ePTFE covering 14 is radially deformed at
body temperature, thereby releasing the strain exerted by the ePTFE
covering 14 on the intraluminal stent 20, permitting the
self-expanding intraluminal stent 20 to radially expand to a
relatively larger diametric dimension D.sub.2.
The fourth preferred embodiment of the present invention consists
generally of a self-expanding intraluminal stent 20 made from
either an elastic spring material or a pseudoelastic shape memory
material, which is at least partially encapsulated in a
substantially monolithic ePTFE covering 14 while in a relatively
larger diametric dimension D.sub.2 such that the ePTFE
encapsulating covering 14 restrains the intraluminal stent from
further self-expansion. The assembly is then worked, such as by
crimping, calendering, folding, compressing or the like to reduce
its diametric dimension to the reduced diametric dimension D.sub.1,
suitable for endoluminal delivery and constrained by an external
constraining sheath 22. Once positioned at a desired intraluminal
site, the constraining sheath 22 is removed to release the
constraining force and the intraluminal stent is permitted to
elastically expand as denoted by arrows 24, carrying the ePTFE
covering 14 into contact with the intraluminal tissue (not
shown).
As will be illustrated by the following examples and the
accompanying process flow diagrams at FIGS. 9A and 9B, the methods
for making each of the foregoing embodiments differ with each
preferred embodiment. The difference in the methods is largely due
to the selection of intraluminal stent type and whether the
intraluminal encapsulated stent is intended for intraluminal
delivery by balloon deformation of the ePTFE covering, whether
delivery will occur due to the self-expanding property of the
intraluminal encapsulated stent and non-deformation of the ePTFE
covering or whether both delivery methods will be employed in
succession.
First Embodiment
In accordance with a first preferred embodiment, illustrated in
FIGS. 2A and 2B, there is provided a balloon expandable
encapsulated shape memory alloy intraluminal stent 10. The balloon
expandable encapsulated shape memory alloy intraluminal stent 10
consists generally of an endoluminal stent 12 fabricated of a shape
memory alloy, preferably one having an A.sub.s value at a
physiologically acceptable temperature compatible with tissue
conservation, such as equiatomic nickel-titanium alloys known as
Nitinol. The endoluminal stent 12 is at least partially
encapsulated in a substantially monolithic ePTFE covering 14 while
the endoluminal stent 12 is in a relatively smaller diametric
dimension D.sub.1. The substantially monolithic ePTFE covering 14
is a continuous integral tubular structure, is free of seams and
covers at least part of both the luminal and abluminal surfaces
about an entire circumferential section of the endoluminal stent 12
along at least a portion of the longitudinal axis of the
intraluminal stent 12. As illustrated in FIGS. 5 and 6, the
substantially monolithic ePTFE covering 14 is characterized by
having a node and fibril microstructure where the nodes are
oriented generally perpendicular to the longitudinal axis 30 of the
stent 12 and the fibrils are oriented generally parallel to the
longitudinal axis 30 of the stent 12, with the distance between
adjacent nodes being termed the "internodal distance." As more
fully described in U.S. Pat. Nos. 5,749,880 and 6,124,523, which
are incorporated by reference, the substantially monolithic ePTFE
covering 14 is preferably radially deformable at applied pressures
less than about six atmospheres, most preferably less than about
three atmospheres, due to the deformable nature of the nodes along
their longitudinal axis, i.e., radial relative to the substantially
monolithic ePTFE covering 14 and perpendicular to the longitudinal
axis 30 of the intraluminal stent 12. The encapsulated intraluminal
stent 10 is radially expandable in vivo under the influence of a
radially outwardly directed force, such as from a balloon catheter,
which radially deforms the ePTFE covering to a second relative
large diametric dimension D.sub.2, to release the constraining
stress exerted on the intraluminal stent by the ePTFE covering
while the encapsulated intraluminal stent 10 is at body
temperature. The simultaneous release of the constraining force
exerted by the ePTFE covering permits the intraluminal stent 12 to
undergo thermomechanical deformation to a larger diametric
dimension.
EXAMPLE 1
Balloon Assisted Thermally Deployed Stent
A balloon assisted encapsulated shape memory alloy stent was
constructed by longitudinally slitting about 5 cm of a 60 cm length
of a first seamless unsintered expanded PTFE tube having an inner
diameter of 3.0 mm. The slit ends were gripped into a fixture
allowing the tube to hang vertically. At the opposite end of the
tube, a length of wire was attached to assist in threading the
tubing through the inner diameter of the stent. The thickness of
the ePTFE layer was measured to be about 0.35 mm using a snap
gauge. The ePTFE tube exhibited a node-fibril microstructure in
which the fibrils were oriented parallel to the longitudinal axis
of the tube throughout the wall thickness of the ePTFE tube.
A 10.times.40 mm shape memory endoluminal stent was placed in a
cold, dry environment at approximately -40.degree. C. and
compressed about a mandrel having an outer diameter of 4.5 mm by
mechanically deforming the stent to circumferentially conform to
the outer diameter of the mandrel. The compressed stent was then
removed from the cold, dry environment and concentrically passed
over the outer diameter of the vertically hanging ePTFE tube,
passing the wire through the stent lumen to assist in engaging the
stent about the abluminal surface of the ePTFE tube without tearing
or marring the ePTFE tube. A 3.3 mm diameter mandrel was then slid
into the lumen of the ePTFE tube/stent assembly, and the tubing was
secured to the mandrel using 1/2 inch strips of tetrafluoroethylene
(TFE) tape. The assembly was then removed from the vertical hanging
fixture.
A 60 cm length of a second seamless partially sintered ePTFE tube,
having an inner diameter of 4.3 mm, slightly larger inner diameter
than the outer diameter of the first ePTFE tube to provide an
interference fit between the first and second ePTFE tubes, was slit
longitudinally in the same manner as described above, and placed in
the vertical hanging fixture. The mandrel bearing the first ePTFE
tube and the shape memory stent was then passed into the lumen of
the second tube, until the stent was approximately centered on the
mandrel. The wall thickness of the second layer was measured as
described above, and the thickness was found to be about 0.35 mm.
Again, as with the first inner ePTFE layer, the fibrils were
oriented parallel to the longitudinal axis of the tube. The ends of
the second tube were also wrapped with strips of TFE tape to secure
to the mandrel.
The assembly was then placed in a helical winding wrapping machine
which tension wraps the assembly with a single overlapping layer of
1/2 inch TFE tape. The overlap of the winding was about 70%. The
tension exerted by the TFE wrapping tape compressed the
ePTFE/stent/ePTFE composite structure against the mandrel, thereby
causing the layers of ePTFE to come into intimate contact through
the interstices of the shape memory stent. The tension wrap was set
to exert 1.7 psi pressure circumferentially around the
ePTFE/stent/ePTFE and mandrel assembly.
The wrapped assembly was placed into a radiant heat furnace, which
had been preheated to a 337.degree. C. set point. The assembly
remained in the furnace for about 7 minutes, and was removed. The
heated assembly was allowed to cool for a period of time sufficient
to permit manual handling of the assembly. After cooling, the TFE
helical wrap was unwound from the sample and discarded. The ePTFE
encapsulated stent assembly was then concentrically rotated about
the axis of the mandrel to release any adhesion between the inner
ePTFE layer and the mandrel. The ePTFE encapsulated stent assembly,
still on the mandrel, was placed into a laser trimming fixture to
trim excess ePTFE materials away from the proximal and distal ends.
After trimming, the trimmed encapsulated stent was removed from the
mandrel.
Five encapsulated stent samples were prepared in accordance with
the foregoing description and were each placed on a 10 mm by 4 cm
PTA balloon dilation catheter. The device was then placed into a
temperature controlled water bath maintained at 37.degree. C. The
balloon was pressurized using a saline filled inflator, thereby
expanding the encapsulated stent. Each encapsulated stent device
was radially expanded under the influence of balloon deformation of
the ePTFE encapsulating covering with full radial deformation to a
10 mm inner diameter occurring at inflation pressures between 2 and
4 atmospheres.
EXAMPLE 2
Thin Wall Thermally Deployed Stent
The radially expanded encapsulated stents obtained from Example 1
were placed over a 10 mm diameter stainless steel mandrel, and
spiral wrapped using 1/2 inch ePTFE tape as described above.
The wrapped assembly was placed again into a radiant heat furnace,
which had been preheated to 337.degree. C. set point. The assembly
remained in the furnace for about 10 minutes and was removed. The
heated assembly was allowed to cool for a period of time sufficient
to permit manual handling of the assembly. After air cooling, the
ends of the mandrel was engaged in two rings and the TFE helical
wrap was unwound from the encapsulated stent samples and discarded.
The encapsulated stents were concentrically rotated about the axis
of the mandrel to release adhesion between the luminal ePTFE
surface of the encapsulated stent and the mandrel.
The encapsulated stent was next cooled to about -20.degree. C. in a
cold dry environment and allowed to equilibrate for 30 minutes. The
cooled encapsulated stent was then rolled between 2 plates to
successively reduce the encapsulated stent inner diameter to about
3.5 mm, representing a reduction of about 40% from the radially
expanded inner diameter of the encapsulated stent. The encapsulated
stent at the reduced inner diameter of 3.5 mm was fully inserted
into a constraining sheath having an inner diameter of
approximately 3.7 mm.
The externally constrained encapsulated stent was then removed from
the cold environment, and placed into a water bath maintained at a
temperature of 37.degree. C. A pusher rod was inserted into the
constraining sheath and impinged upon one end of the, constrained
encapsulated stent. By passing the pusher rod through the
constraining sheath, the encapsulated stent was ejected from the
constraining sheath. As the stent was ejected, it radially dilated
from its compressed state, and re-assumed the original fully
expanded diametric dimension of about 10 mm inner diameter.
Second Embodiment
The second preferred embodiment of the present invention, depicted
in FIGS. 3-4, consists generally of an intraluminal stent 20 made
of a shape memory alloy which is at least partially encapsulated in
a substantially monolithic ePTFE covering 14 while in a relatively
larger diametric dimension D.sub.2 and in the austenite phase,
which is thermomechanically deformed to a temperature induced
martensite phase and to a smaller diametric dimension D.sub.1, and
constrained by constraining sheath 22 for endoluminal delivery.
Once at the delivery site, the constraint 22 is removed and the
intraluminal stent 20 undergoes martensitic transformation to the
austenite state and thermoelastically deforms 24 to its enlarged
diametric dimension D.sub.2 while unfolding the ePTFE covering 14
into contact with the luminal tissue (not shown).
EXAMPLE 3
Thermally Self-Deploying Encapsulated Stent
A thermally deployed encapsulated shape memory alloy stent was
constructed by placing a 40 cm length of a first seamless expanded
PTFE tube over a 10 mm cylindrical stainless steel mandrel. The
inner diameter of the ePTFE tube was of a sufficient size to permit
an interference fit with the mandrel. The thickness of the ePTFE
layer was measured to be about 0.20 mm by taking a radial slice of
the seamless tube, and evaluated by light microscopy incorporating
a calibrated reticle. The ePTFE tube has a node-fibril
microstructure in which the fibrils are oriented perpendicular to
the longitudinal axis of the mandrel throughout the wall thickness
of the ePTFE tube. The ends of the ePTFE tube were wrapped with TFE
tape to keep the tube from sliding along the mandrel for the next
assembly step. A shape memory alloy stent having a nominal inner
diameter of about 10 mm and being about 100 mm in length in its
enlarged diametric configuration was concentrically placed over the
ePTFE covered mandrel at about 22.degree. C. and positionally
centrally along the longitudinal length of the ePTFE tube. The
inner diameter of the shape memory stent was toleranced to the
outer diameter of the ePTFE tube on the mandrel and engaged about
the ePTFE tube without tearing or disturbing the surface of the
ePTFE tube. A second seamless ePTFE tube having a wall thickness of
0.20 mm, measured as described above, was concentrically engaged
over the stent and the first ePTFE tube by first making
diametrically opposed longitudinal slits in one end of the second
ePTFE tube and concentrically inserting the mandrel/first ePTFE
tube/stent assembly into the lumen of the second tube. Again, as
with the first ePTFE tube, the second ePTFE tube exhibited a
node-fibril microstructure in which the fibrils were oriented
parallel to the longitudinal axis of the second ePTFE tube
throughout the wall thickness of the second ePTFE tube. The
opposing ends of the second ePTFE tube were secured about the first
ePTFE tube and the mandrel by tension wrapping with strips of TFE
tape.
The entire assembly was then placed in a helical winding tension
wrapping machine, which tension wrapped the assembly with a single
overlapping layer of 1/2 inch TFE tape in the same manner as in
Example 1 to compress the ePTFE material from the first and second
ePTFE tubes into intimate contact with one another through the wall
openings of the stent. The wrapped assembly was placed into a
radiant heat furnace, which had been preheated to about a
337.degree. C. set point. The assembly remained in the furnace for
about 10 minutes, and was removed. The heated assembly was allowed
to cool for a period of time sufficient to permit manual handling
of the assembly. After cooling, the ends of the mandrel were
engaged in two rings and the TFE helical wrap was unwound from the
encapsulated stent assembly and discarded. The encapsulated stent
assembly was then circumferentially rotated about the axis of the
mandrel to break any adhesion "occurring between the luminal ePTFE
material and the mandrel. Excess ePTFE material from the proximal
and distal ends of the encapsulated stent assembly was then laser
trimmed in the manner described in Example 1 and the encapsulated
stent assembly was removed from the mandrel.
The encapsulated stent was then cooled to about -20.degree. C. in a
cold dry environment and allowed to equilibrate for 30 minutes. The
encapsulated stent was then flattened between 2 plates to bring
diametrically opposed luminal wall surfaces of the encapsulated
stent into contact with one another, thereby creating a flat
structure without an inner lumen. The encapsulated stent was then
folded over itself along its longitudinal axis once, and then again
for a total of one flattening operation and two folding operations.
Thus, the diameter of the embedded stent was reduced about 60% from
its original post-encapsulated diameter. While still in the cold,
dry environment, the device was fully inserted into a constraining
sheath with an internal diameter of approximately 4.7 mm.
The folded and sheathed stent was then removed from the cold
environment and placed into a water bath maintained at a
temperature of 37.degree. C. A pusher was inserted into the lumen
of the constraining sheath and the encapsulated stent was ejected
from the constraining sheath as described above in Example 2. As
the stent was ejected, it unfolded from its flattened and folded
state, and re-assumed the original tubular diametric configuration
having a nominal inner diameter of 10 mm.
Third Embodiment
The third preferred embodiment of the present invention, depicted
in FIGS. 2A-2B, consists generally of self-expanding intraluminal
stent 12 made from either an elastic spring material or of a
pseudoelastic shape memory material, and is at least partially
encapsulated in a substantially monolithic ePTFE covering 14 while
in a relatively small diametric dimension D.sub.1, such that the
ePTFE encapsulating covering 14 acts to impart strain upon the
intraluminal stent 12 and constrain the intraluminal stent 12 from
radial expansion to a relatively larger diametric dimension
D.sub.2. Until it is intraluminally delivered and the ePTFE
encapsulation 14 radially deformed at body temperature to release
the strain exerted by the ePTFE covering 14, the self-expanding
intraluminal stent 12 cannot radially deform to a relatively larger
diametric dimension D.sub.2.
EXAMPLE 4
Elastic Spring Balloon Deployed Encapsulated Stent
An encapsulated elastically self-expanding stainless steel stent
was constructed by placing a 30 cm length of seamless unsintered
ePTFE tube over a 3.3 mm cylindrical stainless steel mandrel. The
inner diameter of the ePTFE tube was toleranced to provide a slight
interference fit to the mandrel. The thickness of the ePTFE layer
was measured to be about 0.35 mm by direct measurement of seamless
tube wall using a snap gauge. The ePTFE tube exhibited a
node-fibril microstructure in which the fibrils were oriented
parallel to the longitudinal axis ePTFE tube throughout the wall
thickness of the ePTFE tube. The ends of the ePTFE tube were
wrapped with strips of TFE tape to retain the position of the ePTFE
tube on the mandrel for the next assembly step. A second seamless
sintered ePTFE tube was concentrically engaged over the first ePTFE
tube by first longitudinally slitting opposing ends of the ends of
the second tube, then 30 inserting the mandrel and first ePTFE tube
into the lumen of the second ePTFE tube. One end of the second
ePTFE tube was wrapped with strips of 1/2 inch TFE tape to secure
it to the first ePTFE tube and the mandrel. The wall thickness of
the second ePTFE tube was measured as described above, and the
thickness was found to be about 0.35 mm. As with the first ePTFE
tube, the second ePTFE tube exhibited a node-fibril microstructure
in which the fibrils were oriented parallel to the longitudinal
axis of the ePTFE tube.
An elastic spring stainless steel stent having a nominal inner
diameter of about 15 mm and a length of about 24 mm in its enlarged
diametric configuration was inserted into a constraining sheath to
reduce the inner diameter to about 4.0 mm. A small length of the
stent is left exposed from one end of the constraining sheath. The
constraining sheath containing the radially constrained stent was
inserted over the mandrel and forced between the first and second
ePTFE tubes such that it was positioned intermediate to the first
and second ePTFE tubes. The exposed end of the stent was then
frictionally engaged through the second ePTFE tube wall and the
constraining sheath was retracted, leaving the stent positioned
between the first and second ePTFE tubes. The unsecured end of the
second ePTFE tube was then secured to the first ePTFE tube and the
mandrel with strips of 1/2 inch TFE tape.
The assembly was then placed in a helical winding machine to
tension wrap a single overlapping layer of 1/2 inch TFE tape, and
sintered in a radiant heat furnace, cooled, the TFE tape unwrapped
and the excess ePTFE laser trimmed as described in Example 1 above.
The resulting encapsulated stent was placed over the balloon on a
12 mm by 4 cm PTA balloon dilation catheter. The device was then
placed into a temperature controlled water bath maintained at
45.degree. C. The balloon was pressurized using a saline filled
inflator which radially deformed the ePTFE encapsulation and
permitted radial expansion of the elastically self-expanding stent.
The encapsulated stent fully radially expanded to a 12 mm inner
diameter at an applied pressure of 2.5 atmospheres.
Fourth Embodiment
The fourth preferred embodiment of the present invention, also
representatively depicted in FIGS. 3-4, consists generally of a
self-expanding intraluminal stent 20 made from either an elastic
spring material or a pseudoelastic shape memory material, which is
at least partially encapsulated in a substantially monolithic ePTFE
covering 14 while in a relatively larger diametric D.sub.2
dimension, such that the ePTFE encapsulating covering 14 acts as to
restrain the intraluminal stent 20 from further self-expansion. The
encapsulated assembly is then worked, such as by crimping,
calendering, folding, or the like, to its reduced diametric
dimension D.sub.1, to achieve a profile suitable for endoluminal
delivery, and the assembly is then constrained by an external
constraining sheath 22. Once positioned at a desired intraluminal
site, the constraining sheath 22 is removed to release the
constraining force and the intraluminal stent 20 is permitted to
elastically expand 24, carrying the ePTFE covering 14 into contact
with the intraluminal tissue (not shown).
EXAMPLE 5
Stress-Induced Martensite Self-Deploying Encapsulated Stent
A self deploying encapsulated shape memory alloy stent is
constructed by placing a 40 cm length of seamless expanded PTFE
tube over a 10 mm cylindrical stainless steel mandrel. The inner
diameter of the ePTFE tube is closely toleranced to provide a
slight interference fit to the mandrel. The thickness of the ePTFE
layer is measured to be about 0.20 mm by taking a radial slice of
the seamless tube, and evaluated by light microscopy incorporating
a calibrated reticle. The tubing is constructed such that the
fibrils are oriented perpendicular to the longitudinal axis of the
mandrel. The ends of the seamless tube are wrapped with strips of
TFE tape to keep the tube from sliding along the mandrel for the
next assembly step. A shape memory alloy stent about 10 mm inner
diameter by 100 mm in length in its enlarged diametric
configuration is placed over the ePTFE covered mandrel at about
22.degree. C. and centered over the ePTFE layer. The inner diameter
of the shape memory stent is closely toleranced to the outer
diameter of the ePTFE covered mandrel. A second tube of seamless
expanded PTFE is placed over the stent by slitting the ends of the
second tube, and inserting the mandrel, ePTFE tube, and stent
assembly into the second tube. The wall thickness of the second
layer is measured as described above, and the thickness is found to
be about 0.20 mm. Again, as with the first inner ePTFE layer, the
fibrils are oriented perpendicular to the longitudinal axis of the
mandrel. The ends of the second tube are also wrapped with strips
of TFE tape.
The assembly is then placed in a helical winding machine, which
wraps the assembly with a single overlapping layer of 1/2 inch TFE
tape. The overlap of the winding was about 70%. The wrapping
material compresses the ePTFE/stent/ePTFE composite structure
against the mandrel, causing the layers of ePTFE to come into
intimate contact through the interstices of the shape memory
stent.
The wrapped assembly is placed into a radiant heat furnace, which
is preheated to a 337.degree. C. set point. The assembly remains in
the furnace for about 10 minutes, and is removed. The heated
assembly is allowed to cool for a period of time sufficient to
permit manual handling of the assembly. After cooling, the ends of
the mandrel are engaged in two rings, allowing the TFE helical wrap
to be unwound from the sample and discarded. The ePTFE/stent
assembly is then rotated about the axis of the mandrel to break the
grip of the inner ePTFE layer to the mandrel. The stent sample,
while still on the mandrel, is placed into a fixture to allow for
laser trimming of the ePTFE materials away from the embedded stent.
Trimming operation is performed on both ends of the device. After
trimming, the embedded and trimming stent was removed from the
mandrel.
The encapsulated stent was then rolled between 2 plates, reducing
the diameter of the stent to about 3.5 mm. Thus, the diameter of
the embedded stent was reduced about 40% from its original post
encapsulated diameter. While in the compressed state, the device
was fully inserted into a constraining sheath with an internal bore
of approximately 3.7 mm. The constrained stent was then placed into
a water bath maintained at a temperature of 37.degree. C. A pusher
was inserted into the bore of the sheath, and the stent was ejected
from the constraining sheath: As the stent was ejected, it unfurled
from its flattened and folded state, and re-assumed the original
post-encapsulation tubular diametric configuration.
Fifth Embodiment
In accordance with a fifth preferred embodiment of the inventive
encapsulated stent, an at least partially unsintered tubular
interlayer is interdisposed between the inner and outer ePTFE
layers and adjacent the intraluminal stent along at least a
longitudinal extent thereof. The interlayer member may consist of a
single tubular member which extends along at least a portion of the
longitudinal axis of the intraluminal stent. Alternatively, the
interlayer member may consist of a plurality of ring-like members
positioned along the longitudinal axis of the intraluminal stent
and in spaced-apart relationship from one and other. The interlayer
member may be preferably employed either: i) where at least one of
the inner and outer ePTFE tubular members of the inventive
encapsulated intraluminal stent is fully sintered to assist in
formation of a monolithic joining of the inner and outer ePTFE
tubular members, and/or 2) to serve as a barrier between a
radiopaque marker and the intraluminal stent to insulate against
galvanic corrosion resulting from contact of metal atoms in a
radiopaque marker and metal in an intraluminal stent. The
interlayer member may be employed with any type of intraluminal
stent, i.e., a shape memory alloy which behaves in either a
thermoplastic or pseudoelastic manner, a self-expanding stent in
which radial expansion is a spring tension mediated event, or a
balloon expandable stent.
EXAMPLE 6
Thermally Self-Deploying Encapsulated Stent
A thermally deployed encapsulated shape memory alloy stent was
constructed by placing a 40 cm length of a first sintered seamless
expanded PTFE tube over a 10 mm cylindrical stainless steel
mandrel. The inner diameter of the ePTFE tube was of a sufficient
size to permit an interference fit with the mandrel. The thickness
of the ePTFE layer was measured to be about 0.20 mm by taking a
radial slice of the seamless tube, and evaluated by light
microscopy incorporating a calibrated reticle. The ePTFE tube
exhibited a node-fibril microstructure in which the fibrils were
oriented parallel to the longitudinal axis of the mandrel
throughout the wall thickness of the ePTFE tube. The ends of the
ePTFE tube were wrapped with TFE tape to keep the tube from sliding
along the mandrel for the next assembly step. A shape memory alloy
stent having a nominal inner diameter of about 10 mm and being
about 100 mm in length in its enlarged diametric configuration was
concentrically placed over the ePTFE covered mandrel at about
22.degree. C. and positionally centrally along the longitudinal
length of the ePTFE tube. The inner diameter of the shape memory
stent was toleranced to the outer diameter of the ePTFE tube on the
mandrel and engaged about the ePTFE tube without tearing or
disturbing the surface of the ePTFE tube. A pair of unsintered
ePTFE rings, prepared by wrapping unsintered ePTFE films (sheets)
concentrically about each of the opposing ends of the shape memory
alloy stent and the first sintered ePTFE tube, were wrapped such
that the node and fibril microstructure of the unsintered ePTFE
rings had a fibril orientation perpendicular to the fibril
orientation of the first ePTFE tube and the longitudinal axis of
the stent.
A second sintered seamless ePTFE tube, having a wall thickness of
0.20 mm, measured as described above, was concentrically engaged
over the entire length of the stent, the pair of unsintered ePTFE
rings and the first ePTFE tube by first making diametrically
opposed longitudinal slits in one end of the second ePTFE tube and
concentrically inserting the mandrel/first ePTFE tube/stent
assembly into the lumen of the second tube. Again, as with the
first ePTFE tube, the second ePTFE tube exhibited a node-fibril
microstructure in which the fibrils were oriented parallel to the
longitudinal axis of the second ePTFE tube throughout the wall
thickness of the second ePTFE tube. The opposing ends of the second
ePTFE tube were secured about the first ePTFE tube and the mandrel
by tension wrapping with strips of TFE tape.
The entire assembly was then placed in a helical winding tension
wrapping machine which tension wrapped the assembly with a single
overlapping layer of 1/2 inch TFE tape in the same manner as in
Example 1 to compress the ePTFE material from the first and second
ePTFE tubes into intimate contact with one another through the wall
openings of the stent.
The wrapped assembly was placed into a radiant heat furnace, which
had been preheated to about a 337.degree. C. set point. The
assembly remained in the furnace for about 10 minutes and was
removed. The heated assembly was allowed to cool for a period of
time sufficient to permit manual handling of the assembly. After
cooling, the ends of the mandrel were engaged in two rings and the
TFE helical wrap was unwound from the encapsulated stent assembly
and discarded. The encapsulated stent assembly was then
circumferentially rotated about the axis of the mandrel to break
any adhesion occurring between the luminal ePTFE material and the
mandrel. Excess ePTFE material from the proximal and distal ends of
the encapsulated stent assembly was then laser trimmed in the
manner described in Example 1 and the encapsulated stent assembly
was removed from the mandrel.
The encapsulated stent was then cooled to about -20.degree. C. in a
cold dry environment and allowed to equilibrate for 30 minutes. The
encapsulated stent was then flattened between 2 plates to bring
diametrically opposed luminal wall surfaces of the encapsulated
stent into contact with one another, thereby creating a flat
structure without an inner lumen. The encapsulated stent was then
folded over itself along its longitudinal axis once, and then again
for a total of one flattening operation and two folding operations.
Thus, the diameter of the embedded stent was reduced about 60% from
it original post encapsulated diameter. While still in the cold,
dry environment, the device was fully inserted into a constraining
sheath with an internal diameter of approximately 4.7 mm.
The folded and sheathed stent was then removed from the cold
environment, and placed into a water bath maintained at a
temperature of 37.degree. C. A pusher was inserted into the lumen
of the constraining sheath and the encapsulated stent was ejected
from the constraining sheath as described above in Example 2. As
the stent was ejected, it unfolded from its flattened and folded
state, and re-assumed the original tubular diametric configuration
having a nominal inner diameter of 10 mm.
While the interlayer member employed in the foregoing Example 6
were rings produced from wrapping sheets of unsintered ePTFE
material, it will also be appreciated that tubular or ring-like
unsintered ePTFE members may be employed. Where the interlayer
member is a tubular or ring-like unsintered ePTFE member, the
interlayer member will preferably have a node and fibril
microstructure in which the fibril orientation of the interlayer
member is parallel to the longitudinal axis of the interlayer
member and parallel with the fibril orientation of the inner and
outer ePTFE tubular members which the interlayer member is
interdisposed between. This co-parallel arrangement of the fibril
orientations of the interlayer member and the inner and outer ePTFE
tubular members permits the resulting encapsulated stent device to
be further radially expanded by balloon expansion in order to
further model the in vivo profile to the receiving anatomical
structure at radial expansion pressures comparable to that of the
balloon assisted encapsulated stent embodiments described
above.
Where a thermoplastic transformation of a shape memory intraluminal
stent is desired, care must be taken to: 1) avoid imparting a
secondary shape memory to the shape memory alloy during sintering
of the ePTFE encapsulating covering, 2) avoid inducing
stress-induced martensite formation during thermomechanical forming
for either encapsulation or mounting onto a delivery catheter, and
3) avoid inducing non-recoverable strains by exceeding the strain
limit of the shape memory alloy material used. Where the elastic
behavior of a stent made of either a pseudoelastic shape memory
alloy or a spring tension material, care must be taken to avoid
plastically deforming the stent which would deleteriously effect
the elastic deformation property of the intraluminal stent during
intraluminal delivery. Finally, where the pseudoelastic behavior of
an intraluminal stent made of a shape memory material is to be
utilized in the encapsulated intraluminal stent, care must be taken
to maintain the temperature of the shape memory alloy above
A.sub.f, but below M.sub.d during either the process of
encapsulating the stent at a reduced diameter, and before
sintering, for balloon expansion in vivo or during deformation of
the encapsulated stent to a reduced delivery diameter for loading
onto a delivery catheter. In this manner, the stress-induced
martensite phase will be induced in the shape memory alloy during
deformation of the stent to a diametric dimension suitable for
endoluminal delivery and maintained so that when the encapsulated
stent, in the stress-induced martensite state, is delivered and
either the ePTFE constraint or the constraining sheath is relieved,
the strain is released and the stent, in the stress-induced
martensite phase, is permitted to transform to austenite and the
stent to elastically deform to its pre-programmed diametric
dimension.
The methods described in the foregoing Examples are summarized in
FIGS. 9A-9B, which are process flow diagrams setting forth the
fundamental method steps of the methods to make each of the
above-described preferred embodiments. Where a shape memory
intraluminal stent is to be encapsulated in an ePTFE covering and
thermoplastic transformation of the shape memory stent is desired,
either in a balloon assisted expandable encapsulated stent
embodiment or in a self-expanding encapsulated stent embodiment,
the thermoplastic deformation of the shape memory stent from its
enlarged diametric dimension D.sub.2, to its reduced diametric
dimension D.sub.1, may be accomplished in accordance with the
method 40 set forth in FIG. 9A. Thermoplastic deformation method 40
entails first providing an shape memory alloy intraluminal stent
having a predetermined shape memory configuration 42. The
intraluminal stent is then exposed to a temperature below the
martensite transformation temperature M.sub.s of the shape memory
alloy 44 and allowed to equilibrate at the sub-martensite
transformation temperature M.sub.s 46 While still below the M.sub.s
temperature, the stent is mechanically deformed 48 to reduce its
diameter from the enlarged diametric dimension D.sub.2 to a reduced
diametric dimension D.sub.1 suitable for endoluminal delivery. The
stent at its reduced diametric dimension 50 is now at a dimensional
state suitable for encapsulation at its reduced diametric dimension
D.sub.1.
The encapsulation method 60 is more fully set forth in FIG. 9B, and
is applicable for either a shape memory alloy intraluminal stent
which is to be encapsulated either at its reduced diametric
dimension D.sub.1, or at its enlarged diametric dimension D.sub.2,
as well as for a self-expanding stent which radially expands due to
inherent spring tension in the stent. A luminal ePTFE tube 62 is
concentrically engaged upon a mandrel 64 and secured to the
mandrel. Either a shape memory stent 52 or a self-expanding stent
80 is selected at step 66. If a shape memory stent is selected 70,
the shape memory stent 52 is engaged over the luminal ePTFE tube at
step 54 while maintaining the stent at a temperature below A.sub.s
to prevent the stent. from radially expanding. If a self-expanding
stent 80 is selected 68, an abluminal ePTFE tube is concentrically
engaged over the luminal ePTFE tube and the self-expanding stent 80
interdisposed between the luminal and abluminal ePTFE tubes and
secured there between 78. Where a shape memory alloy intraluminal
stent is employed 74, the abluminal ePTFE tube is concentrically
engaged over the stent. Once the stent is positioned intermediate
between the luminal and abluminal ePTFE tubes, the entire assembly
is then wrapped with TFE tape 82 to exert a circumferential
pressure about the entire circumference of both the luminal and
abluminal ePTFE tubes and the stent, causing the ePTFE tubes to be
motivated into intimate contact with one and other through the
interstices of the stent. The entire wrapped assembly is then
sintered 84 and excess ePTFE overlaying ends of the stent may be
trimmed 86.
Once trimmed, the encapsulated stent is then prepared for mounting
onto a delivery catheter 88, either by mounting the encapsulated
stent in its reduced diametric dimension D.sub.1 onto a balloon
catheter for balloon-assisted delivery, or by thermomechanical
deformation from the enlarged diametric dimension D.sub.2 to the
reduced diametric dimension D.sub.1, following the method steps of
thermomechanical deformation 40 or formation of stress-induced
martensite for pseudoelastic recovery by crimping, folding or
otherwise reducing the encapsulated stent to its reduced diametric
dimension D.sub.1, mounting onto a delivery catheter and applying
an external constraining sheath concentrically over the
encapsulated stent.
Those skilled in the art will understand and appreciate that while
the present invention has been described with reference to its
preferred embodiments and the examples contained herein, certain
variations in material composition, shape memory alloy
constitution, stent and ePTFE dimensional size and configuration,
temperatures, times and other operational and environmental
conditions may be made without departing from the scope of the
present invention which is limited only by the claims appended
hereto. For example, one skilled in the art will understand and
appreciate from the foregoing that the methods for making each of
the foregoing embodiments differs with each preferred embodiment.
These differences in the methods are largely due to the selection
of intraluminal stent type and whether the intraluminal
encapsulated stent is intended for initial intraluminal delivery by
balloon expansion or whether initial delivery will occur due to the
self-expanding property of the intraluminal encapsulated stent.
* * * * *